English holly identification and control

Ilex aquifolium

English holly is a broadleaf evergreen tree or shrub that is grown as a landscape plant in the northwestern United States and Canada but is also commonly found escaping into forests in this region. English holly's native range is the British Isles to southern and central Europe. It is grown commercially in the Pacific Northwest and commonly used in decorations and floral arrangements as well as in landscapes. See Holly Research and Reports for more information and research reports on this species.

Legal status in King County, Washington

In King County, English holly is classified as a Weed of Concern and its control is recommended in natural areas that are being restored to native vegetation and in protected forest lands. The City of Kirkland includes English holly on its list of prohibited plants that are prohibited from being planted on private property.
English holly is not on the Washington State Noxious Weed List, although it is on the monitor list of the Washington State Noxious Weed Control Board(external link). For more information about noxious weed regulations and definitions, see Noxious weed lists and laws.

Description

Large, dense, slow-growing, evergreen tree or shrub, 15 to 50 feet tall and up to 15 feet wide or more

Leaves usually have sharp, stout spines along edges although may be smooth on older branches

Flowers are small, whitish, inconspicuous, sweetly scented

Bunches of red, yellow or orange berries, poisonous to people but not to birds, borne on female trees in winter

Grows in shade or sun in well drained soil

Creates deep shade under its canopy

Reproduction and spread

English holly is pollinated by bees so female trees must grow within bee range (100 feet or so) of a male to be pollinated

Holly berries are dispersed by birds

Also spreads by suckering and layering

Impacts and distribution

English holly is carried by birds into forests where it can form dense thickets that dominate the tall shrub layer and suppress germination and growth of native tree and shrub species. According to the USDA distribution map, English holly is reported as naturalized in Washington, Oregon, California, and Hawaii. Here in Washington, holly is mostly limited to western counties.

According to the University of Washington Herbarium records, botanists have collected specimens of English holly from naturalized populations in 17 western Washington counties, including King County. Collected plants have varied from immature plants to tall trees, and both male and female plants have been found. In some cases, the holly was growing as a single tree or in small, patchy populations, but in other cases, there were substantial populations noted that included a range of ages from seedlings to fruiting trees. The locations of the collections also ranged from urban to rural, sometimes near where people probably planted it intentionally, but also in areas far from any intentional plantings, most likely brought there by birds. Many of the records indicate that English holly was naturalized at the site and growing with mostly native Washington forest species or a mix of native and non-native plants. It does appear that English holly is encroaching into native forest habitat and reproducing successfully in fairly undisturbed native communities.

The Middle Fork Snoqualmie invasive plant inventory undertaken by the Mountains to Sound Greenway Trust discovered significant populations of English holly in the forests of the watershed, particularly in the State DNR lands on the western edge of the watershed, but also farther up in the watershed away from any residential properties. There doesn’t seem to be a limit to how far into a forest holly can penetrate and it seems very well suited to our climate and our soils. Fortunately, most populations in the watershed are scattered at this point and it is probably not too late to stop them from spreading much further.

In the Seattle Urban Nature Project’s (now EarthCorps) plant inventory of Seattle’s public forests, English holly was frequently found in the understory. In fact, English holly was the fourth most abundant non-native species found, outnumbered only by Himalayan blackberry, Scotch broom and English ivy. English holly, along with English laurel, was more common in the understory than native conifers. Given their findings, it is likely that English holly, along with other invasive non-natives, will be in a much better position to replace Seattle’s aging deciduous trees than our native evergreen trees. Seattle Urban Nature ecologist Ella Elman predicts that, if nothing is done, 30 or 40 years from now Seattle’s forests will look dramatically different than they do today.

Control

Mature trees have deep and extensive roots so digging is labor-intensive and results in considerable soil disturbance if all of the roots are removed.

Cutting holly at the base usually results in re-sprouting from the crown, but with monitoring and follow up this can suppress the holly.

Applying herbicide with the cut stump or frilling method are most effective. Foliar herbicide treatment is not very effective due to the thick, waxy leaves.

What to do if you find this plant

Because English holly is already naturalized in many places in King County and is not on the State Noxious Weed List, we are not tracking locations. However, if you know of any heavily infested forests, especially outside of the urban growth area, we would be interested in having that information. We are also gathering information on effective control methods. If you have had any success (or failures) controlling this plant, please contact our office.

It is important to stress that English holly is produced and sold in Washington and Oregon and there are no state or countywide regulations or limits on its sale or use in landscaping. However, some cities have more restrictive codes related to invasive plants and should be consulted for more information. For instance, the City of Kirkland has a list of prohibited plants that are prohibited from being planted on private property, including English holly.